Obesity and its complications are major health problems in black women. This initial study will serve two functions: establish a link between the Obesity Research Center and the black community; and provide a foundation for future studies that will examine sociologic and physiologic mechanisms leading to weight gain and the efficacy of weight reduction therapy. Quantifying total body fat and fat free body mass (FFM) is central to the study of obesity. Fat reflects the primary determinant of excess weight, adipose tissue, while FFM is used in adjusting resting metabolic rate (RMR) for between subject comparisons. There are 3 benchmark methods used in subdividing weight into these 2 compartments; hydrodensitometry, total body water (TBW), and total body potassium (TBK). Each relies on an experimentally determined coefficient for lean density (DFFM), TBW/FFM, and TBK/FFM, respectively. These coefficients were developed in a small number of white cadavers. Early studies suggested blacks have relatively more skeletal muscle and bone than whites. Using newly developed in vivo methods (dual photon absorptiometry (DPA); delayed-delta, prompt-delta, and inelastic scattering neutron activation) we confirmed these earlier findings in a small pilot study and also evolved a new method of deriving DFFM. These results demonstrated blacks and whites differ in lean density, TBK/FFM, and potentially TBW/FFM. Two-compartment models are thus inaccurate in blacks when coefficients developed in whites are used. A second concern relates to use of 2 compartment coefficients in the obese; previous studies indicate adipose-tissue related expansion of extracellular fluid and consequent increase in TBW/FFM and decrease in DFFM and TBK/FFM. The aims of the present study are fourfold: to confirm black-white body composition differences; to establish to what extent obesity alters lean hydration; to develop new RMR-body composition models based upon active cell mass; and to apply RMR models in comparing blacks and whites. A case-comparison design is planned in which black women (n=60), ages 20-60 yrs, are matched to white women by age, weight, height, and menstrual status. The 120 subjects will be evenly divided between lean and obese. Each will undergo hydrodensitometry, anthropometry, RMR, DPA, TBW, TBK, and neutron activation. These multiple methods will allow us to accomplish aims by providing models of body composition and energy expenditure that are independent from ethnicity and obesity.